In systems for transmitting moving picture signals to remote place, such as, for example, television conference system or television telephone system, etc., in order to efficiently utilize transmission path, picture signals are caused to be compression-coded by making use of line correlation or interframe correlation of video signal. As such compression encoding system, there are so called MPEG1 system and MPEG2 system (hereinafter encoding system including both systems will be called MPEG system).
When line correlation is utilized, picture signals are caused to undergo, e.g., DCT (Discrete Cosine Transform) processing, etc., thus making it possible to compress them.
Moreover, when interframe correlation is utilized, picture signals are caused to be further compressed, thus making it possible to encode them. When it is assumed that frame pictures PC1, PC2, PC3 respectively take place at times t1, t2, t3 as shown in FIG. 1A, for example, difference of picture signal between frame pictures PC1 and PC2 is calculated to generate PC12, and difference between frame pictures PC2 and PC3 is calculated to generate PC23, as shown in FIG. 1B.
Since pictures of frames adjacent in point of time ordinarily have not so great change, when difference therebetween is calculated, its difference signal takes small value. Accordingly, if such difference signal is encoded, code quantity can be compressed.
However, even if only difference signal is transmitted, it is impossible to restore (reconstruct) original picture. In view of this, in MPEG, pictures of respective frames are caused to be any ones of three kinds of I picture, P picture and B picture, thus to compression-encode picture signals.
Namely, as shown in FIGS. 2A and 2B, for example, picture signals of 17 frames of frames F1 to F17 are caused to be group of pictures, and this group of pictures is used as unit of processing. Picture signal of the leading frame F1 is encoded as I picture, the second frame F2 is processed as B picture, and the third frame F3 is processed as P picture. Further, the fourth frame and frames succeeding (subsequent) thereto which are labeled F4 and F17 are alternately processed as B picture and P picture.
As picture signal of I picture, picture signal of one frame is transmitted as it is. On the contrary, as picture signal of P picture, basically, difference from picture signal of I picture or P picture preceding thereto in point of time is transmitted as shown in FIG. 2(A). Further, as picture signal of B picture, basically, difference from mean value of both frames of frame preceding in point of time and frame succeeding in point of time is obtained as shown in FIG. 2(B) to encode its difference.
FIGS. 3A and 3B show the principle of method of encoding moving picture signal. As shown in the figure, since the first frame F1 is processed as I picture, it is transmitted to transmission path as transmit data F1X as it is (intra-frame coding). On the contrary, since the second frame F2 is processed as B picture, difference from mean value of frame F1 preceding in point of time and frame F3 succeeding in point of time is calculated, and the difference thus calculated is transmitted as transmit data F2X.
When further detailed explanation is given, there exist, in macro blocks, four kinds of processing as B picture as mentioned above. The first processing transmits data of original frame F2 as transmit data F2X as it is (SP1) (Intra Coding), and is a processing similar to the case in I picture. The second processing calculates difference from frame F3 succeeding in point of time to transmit its difference (SP2) (Backward Predictive Coding). The third processing transmits difference (SP3) from frame F1 preceding in point of time (Forward Predictive Coding). In addition, the fourth processing generates difference (SP4) from mean value of frame F1 preceding in point of time and frame F3 succeeding in point of time to transmit it as transmit data F2X (Bidirectionally Predictive Coding).
Among these four methods, a method in which quantity of data transmitted becomes minimum is adopted.
In this case, at the time of transmitting difference data, motion vector x1 with respect to picture (predictive picture) of frame subject to calculation of difference (motion vector between frames F1 and F2) (case of forward prediction), x2 (motion vector between frames F3 and F2) (case of backward prediction), or both x1 and x2 (case of bidirectional prediction) is or are transmitted along with difference data.
Moreover, with respect to frame F3 of P picture, with frame F1 preceding in point of time being as predictive picture, difference signal (SP3) between the frame F3 and the frame F1, and motion vector x3 are calculated, and they are transmitted as transmit data F3X (Forward Predictive Coding). Alternatively, data of original frame F3 is transmitted as transmit data F3X as it is (SP1) (Intra Coding). With respect to which method is employed in transmission, a method in which transmit data quantity becomes lesser is selected in macro block units similarly to the case of B picture.
FIG. 4 shows an example of the configuration of a system operative on the basis of the above-described principle so as to encode moving picture signal to transmit the coded signal to decode it. Coding unit 1 encodes inputted video signal to transmit it onto recording medium 3 as transmission path. Further, decoding unit 2 reproduces signal recorded on recording medium 3 to decode the reproduced signal to output it.
In the coding unit 1, inputted video signal is inputted to pre-processing circuit 11, at which it is separated into luminance signal and color signal (color difference signal in the case of this example). These signals are caused to respectively undergo A/D conversion by A/D converters 12, 13. Video signals converted into digital signals after undergone A/D conversion by A/D converters 12, 13 are delivered to frame memory 14, and are stored thereinto. Frame memory 14 allows luminance signal frame memory 15 to store luminance signal, and allows color difference signal frame memory 16 to store color difference signal.
Format converting circuit 17 converts signal of frame format stored in frame memory 14 into signal of block format. Namely, as shown in FIG. 5, video signal stored in frame memory 14 is caused to be data of frame format in which V lines comprised of H dots per each line are gathered. Format converting circuit 17 divides signal of one frame into M slices with 16 lines being as a unit.
Each slice is divided into M macro blocks. Each macro block consists of luminance signals corresponding to 16.times.16 pixels (dots). This luminance signal is further divided into blocks Y [1] to Y [4] having 8.times.8 dots as unit. Cb signal of 8.times.8 dots and Cr signal of 8.times.8 dots are caused to correspond to luminance signal of 16.times.16 dots.
Data converted into block format in this way is delivered from format converting circuit 17 to encoder 18, at which encode (encoding) operation is carried out. Since the detail thereof does not affect the subject matter of this invention, its explanation is omitted here.
Signal encoded by encoder 18 is outputted to transmission path as bit stream, and is recorded onto, e.g., recording medium 3.
Data reproduced from the recording medium 3 is delivered to decoder 31 of decoding unit 2, at which such data is decoded. Since the detail of decoder 31 does not affect the subject matter of this invention, its explanation is omitted here.
Data decoded by decoder 31 is inputted to format converting circuit 32, at which it is converted from block format to frame format. Luminance signal of frame format is delivered to luminance signal frame memory 34 of frame memory 33, and is stored thereinto. Color difference signal is delivered to color difference signal frame memory 35, and is stored thereinto. Luminance signal and color difference signal which have been read out from luminance signal frame memory 34 and color difference signal frame memory 35 are caused to respectively undergo D/A conversion by D/A converters 36 and 37, and are then delivered to post-processing circuit 38, at which they are synthesized. Synthesis output thus obtained is outputted to display, e.g., CRT (not shown), etc., and is displayed thereon.
There are instances where coded moving picture signals (bit streams) are transmitted from, e.g., antenna of broadcasting station, and there are also instances where such coded moving picture signals are recorded onto recording media such as digital VTR/digital video disc, etc.
System for recording/reproducing such coded moving picture signals (bit streams) is constituted as shown in FIG. 6, for example, in the prior art. Digital VTR recording/reproducing unit 51 reads out bit stream recorded on digital VTR to decode it to output video/audio signal. Digital video disc unit 52 reads out bit stream recorded on digital video disc to decode it to output video/audio signal. Digital TV receiving tuner 53 receives bit stream which has been transmitted as radio wave to decode it to output video/audio signal.
Video/audio signals outputted from these units are inputted to monitor 54. This monitor 54 is comprised of display, e.g., CRT, etc. to switch input source by using switch, etc., thereby making it possible to display moving picture signal from specified (designated) moving picture reproducing unit.
Moreover, in the case where moving picture signal received by, e.g., digital TV receiving tuner 53 is recorded by digital VTR recording/reproducing unit 51, video/audio signal that digital TV receiving tuner 53 outputs is inputted to digital VTR recording/reproducing unit 51. This digital VTR recording/reproducing unit 51 encodes inputted video/audio signal to write it onto recording media in the form of bit stream.
In the case where such a moving picture reproducing system is constituted as shown in FIG. 6 to connect a plurality of moving picture reproducing units to, e.g., one moving picture display unit to utilize source that user wants to see by selection of switch, decoding units for bit streams are respectively required in reproducing units connected to the system. Since encoding/decoding methods for bit stream are being standardized over the world by system such as MPEG, etc., these decoding units are of entirely the same structure. Accordingly, such system requires decoding units by the number of reproducing units are required, and is disadvantageous in terms of cost.
Furthers in the case where, e.g., moving picture signal reproduced by a certain moving picture reproducing unit is recorded by different moving picture recording unit, signal once decoded is encoded for a second time and is then recorded. Also in this case, it is sufficient that bit steam before subjected to decoding and bit stream after undergone encoding are the same. Accordingly, this system additionally requires encoding unit or units and is therefore disadvantageous in terms of cost. In addition, degradation of picture quality by encoding becomes problem.